Trialkylamines more planar at nitrogen than triisopropylamine in the solid state

Abstract

A short synthesis of exceedingly congested amines, with insertion of a rhodium carbenoid into an N-H bond as the key step, is described. Trialkylamines such as 2-(diisopropylamino)propane-1,3-diol (8) and 2-(2,2,6,6-tetramethyl-1-piperidinyl)propane-1,3-diol (10) may be prepared easily. Examination of amines of this type by X-ray crystallography reveals nearly planar nitrogens. Indeed, they are all more planar than the nitrogen of triisopropylamine, which had long been thought to be perfectly planar, but which is in fact very slightly pyramidal. The distance (h) of the nitrogen of triisopropylamine to the plane defined by the three carbons to which it is bonded is 0.28-0.29 A. In 8, by comparison, h is 0.185 A. A qualitative orbital interaction explanation is proposed to rationalize the tendency of nitrogen to planarize when attached to the 1,3-dihydroxy-2-propyl group. Cyclic voltammetric measurements of nitrogen one-electron oxidation of the nearly planar trialkylamines revealed no correlation between E(1/2)(ox) and degree of planarity.